1. Field of the Invention
The present invention relates generally to a color cathode-ray tube. Particularly, the present invention relates to a color cathode-ray tube including a shadow mask stretched and held while being provided with a tensile force.
2. Related Background Art
In a color cathode-ray tube, a phosphor screen formed on an inner face of a face panel is irradiated with electron beams emitted from an electron gun, thus displaying a desired image. On the electron gun side of the phosphor screen, a shadow mask functioning as a color selection electrode is provided at a predetermined distance from the phosphor screen. In the shadow mask, a number of openings (electron beam through holes) are formed and arranged so that the electron beams strike phosphors at predetermined positions.
When the electron beams strike the shadow mask, the shadow mask expands thermally. This causes displacement of the openings and therefore the electron beams that have passed through the openings no longer strike the phosphors at predetermined positions properly, thus causing color unevenness. Such a phenomenon is called xe2x80x9cdomingxe2x80x9d. In order to prevent this, the shadow mask is stretched and held by a mask frame while being provided with a tensile force that can absorb thermal expansion caused by the temperature increase. Therefore, the relative position shift between the openings of the shadow mask and phosphor stripes formed on the phosphor screen can be reduced even when the temperature of the shadow mask rises.
The mask frame is constructed by assembly of iron members including a pair of opposed supporters for stretching the shadow mask to have a substantially rectangular frame shape. Each supporter has a substantially L-shaped cross-section and the shadow mask is stretched at one end of the supporter in its width direction. In order to apply a predetermined tensile force to the shadow mask, the supporters are required to have sufficient strength for resisting the tensile force.
In this connection, methods conventionally used for manufacturing such supporters include, for example, a method of bending a strip-like iron plate with a predetermined thickness by press processing or roll forming so that the iron plate has a substantially L-shaped cross section with its corner at substantially the center in its width direction, a method of drawing an iron material using a die with a substantially L-shaped opening, or the like.
However, in the method of bending an iron plate by press processing or roll forming, an inner side of a corner formed by the bending is recessed during bending, thus causing the reduction in wall thickness of the corner, which is a phenomenon of a so-called xe2x80x9cthickness lossxe2x80x9d. When the corner of a stress concentration part is subjected to the thickness loss, the strength of the supporter is lowered and thus a desired tensile force cannot be applied to the shadow mask. In order to apply a desired tensile force, it is required to use a thick strip-like iron plate with consideration to the thickness loss during processing, thus increasing weight and cost.
In addition, when a hot rolled steel sheet is used as a strip-like iron plate for the raw material, it has a great unevenness in thickness and therefore a greatly uneven tensile force is applied to the shadow mask. Consequently, a cathode-ray tube with a stable quality cannot be obtained. When the hot rolled steel sheet is subjected to cold rolling before the bending process, the unevenness in thickness is reduced, but the number of steps increases, thus increasing the cost.
On the other hand, in the drawing process, it is difficult to obtain a thin member with a substantially L-shaped cross-section. When the member is thick, it is difficult to obtain a desired tensile force and the weight and the cost increase.
The increase in the weight of the supporters increases the weight of a mask structure formed of the shadow mask to be stretched and the mask frame. As a result, deformation of elastic supporters for holding the mask structure with respect to the face panel increases. In addition, the elastic supporters tend to be deformed permanently due to shocks from the outside or the like. Any of the above-mentioned deformations can cause color unevenness.
It is an object of the present invention to solve the above-mentioned conventional problems in a color cathode-ray tube with a shadow mask stretched and held while being provided with a tensile force. In other words, the present invention is intended to provide an inexpensive color cathode-ray tube having a stable quality and causing less color unevenness, which is obtained using supporters with stable qualities that can apply a desired tensile force to a shadow mask without causing the increases in weight and cost.
In order to achieve the above-mentioned object, the following configurations are employed.
A color cathode-ray tube of a first configuration of the present invention includes a mask frame and a shadow mask functioning as a color selection electrode. The mask frame includes at least a pair of opposed supporters and is formed in a substantially rectangular frame shape. The shadow mask is stretched and held by the supporters while being provided with a tensile force. Each of the supporters is a member with a substantially L-shaped cross-section and is obtained by the forming of a strip plate material having a middle part and thinner thickness-deviation portions on both sides of the middle part by rolling of a metal material and then bending and deforming of the strip plate material in its width direction by roll forming to form a bent and deformed portion in the middle part.
A color cathode-ray tube of a second configuration of the present invention includes a mask frame and a shadow mask functioning as a color selection electrode. The mask frame includes at least a pair of opposed supporters and is formed in a substantially rectangular frame shape. The shadow mask is stretched and held by the supporters while being provided with a tensile force. Each of the supporters is a member with a substantially L-shaped cross-section and is obtained by the forming of a strip plate material by rolling of a metal material, the strip plate material having a middle part, thinner thickness-deviation portions on both sides of the middle part, and a thick stripe portion at a predetermined position in the middle part corresponding to an outer corner of a bent and deformed portion to be formed, and then bending and deforming of the strip plate material in its width direction at a position of the thick stripe portion by roll forming to form the bent and deformed portion.
According to the methods of processing the supporters according to the first and second configurations, the bent and deformed portion obtained by the roll forming has at least the same thickness as an original thickness of the metal material, and the thickness-deviation portions positioned on both sides of the bent and deformed portion are formed to be thin in the rolling step prior to the roll forming. Therefore, the supporters of the present invention can be reduced in weight while maintaining substantially the same strength as that of a conventional supporter with a substantially L-shaped cross-section.
Since the process includes the rolling step and the roll forming step, the processing can be carried out economically in relatively simple steps.
Even if the metal material has unevenness in thickness, the unevenness is corrected during the formation of the thickness-deviation portions in the rolling step, thus unifying the thickness. Therefore, an inexpensive hot roll steel sheet can be used, thus obtaining supporters with stable qualities at a low cost.
Consequently, the shadow mask is stretched and held using the above-mentioned supporters while being provided with a tensile force, thus providing an inexpensive color cathode-ray tube having stable qualities and causing less color unevenness.
The quality and shape of the metal material as a raw material of the supporters are not particularly limited as long as the metal material is a steel material or the like suitable for plastic processing.
In the rolling step for forming the strip plate material from the metal material, the thickness-deviation portions may be formed to be reduced in thickness gradually from the middle part toward the ends of side portions positioned on both sides of the middle part. Furthermore, at a predetermined position in the middle part corresponding to an outer corner of a bent and deformed portion to be formed, a thick stripe portion is formed suitably. The thick stripe portion may be formed of one stripe, but also may be formed of two adjacent protruding stripes extending continuously along the length direction in the middle part of the strip plate material, with the portion between the two protruding stripes being recessed in an arc shape.
For the rolling of the metal material, warm rolling can be employed. Preferably, however, rolling under ordinary temperature (cold rolling) is employed and the next step of roll forming is carried out while processing heat generated during the rolling still remains in the strip plate material. In other words, the rolling step and the roll forming step are carried out successively, and therefore the roll forming can be carried out with high efficiency using the processing heat generated during the rolling.
The strip plate material having the middle part formed to be thicker than the thickness-deviation portions is bent and deformed at a position of the middle portion in its width direction by roll forming, thus forming the bent and deformed portion. Thus, a desired supporter with a substantially L-shaped cross-section is obtained.
It also is possible that the above-mentioned rolling step and roll forming step are not carried out as two separate steps but concurrently at substantially the same time.
A color cathode-ray tube with a third configuration of the present invention includes a mask frame and a shadow mask functioning as a color selection electrode. The mask frame includes at least a pair of opposed supporters and is formed in a substantially rectangular frame shape. The shadow mask is stretched and held by the supporters while being provided with a tensile force. Each of the supporters is a member with a substantially L-shaped cross-section and is obtained by a process in which while thinner thickness-deviation portions are formed on both sides of a middle part of a metal material by rolling of the metal material, the metal material is bent and deformed in its width direction by roll forming to form a bent and deformed portion in the middle part.
A color cathode-ray tube with a fourth configuration of the present invention includes a mask frame and a shadow mask functioning as a color selection electrode. The mask frame includes at least a pair of opposed supporters and is formed in a substantially rectangular frame shape. The shadow mask is stretched and held by the supporters while being provided with a tensile force. Each of the supporters is a member with a substantially L-shaped cross-section and is obtained by a process in which while rolling of a metal material to form thinner thickness-deviation portions on both sides of a middle part of the metal material and a thick stripe portion at a predetermined position in the middle part corresponding to an outer corner of a bent and deformed portion to be formed, the metal material is bent and deformed in its width direction at a position of the thick stripe portion by roll forming to form the bent and deformed portion.
Even when the metal material is bent and deformed by roll forming while the thickness-deviation portions (and further the thick stripe portion as required) are formed by rolling as in the third and fourth configurations, the same effects as in the first and second configurations can be obtained. Moreover, according to the third and fourth configurations, the forming steps can be simplified and the processing heat generated during the rolling can be used for the roll forming effectively, thus improving formation efficiency.
In the first to fourth configurations described above, the supporter may have a tapered face and/or a protruding portion formed at an end in its width direction. In this case, preferably, the tapered face and/or the protruding portion are/is formed by the rolling or the roll forming. Conventionally, when the tapered face and/or the protruding portion are/is formed at an end, an independent step other than the step of forming the bent and deformed portion was necessary. On the other hand, in the present invention, the tapered face and/or the protruding portion also can be formed in the rolling step or the roll forming step, thus simplifying the steps.
Furthermore, in the above-mentioned first to fourth configurations, the pair of supporters may form long sides of the mask frame and the shadow mask may be provided with a tensile force in its short-side direction.